Effect of Morphine Dose Size on the Conditioned Reinforcing Potency of Stimuli Paired with Morphine

Abstract

After an initial operant level period, 22 rats were given non-contingent pairing of a buzzer with intravenous injections of morphine solution in doses of 0.0032 mg/kg, 0.032 mg/kg, and 0.32 mg/kg. The buzzer plus a saline infusion were then presented contingently on bar-pressing behavior. The results of this test indicated that buzzer plus saline infusion had acquired secondary reinforcing properties, the magnitude of which increased with increasing dosage of morphine. The results may be attributed to conditioned positive reinforcement, since no physical dependence was evident. The present findings also suggest that a stimulus can become a secondary reinforcer without being a discriminative stimulus for an operant.